A diagrammatic version would be just a box with circles for wheels. The best thing is to look closely at object and copy again and again until happy with image.
I'm not that great of an artist and my drawings are usually a little rough looking when drawn off the top of my head. Pictures or other drawings that I copy by site or trace over come out much better and I'm almost always satisfied with what's on the paper, be it a car or something else!
Draw two circles four inches apart. Add a rectangle on top of the circles. Add another rectangle to the left and bottom of the first rectangle. It's a car!